Internet Engineering Task Force S. Morris
Internet-Draft ISC
Intended status: Informational J. Ihren
Expires: January 10, 2013 Netnod
J. Dickinson
Sinodun
July 9, 2012
DNSSEC Key Timing Considerationsdraft-ietf-dnsop-dnssec-key-timing-03.txt
Abstract
This document describes the issues surrounding the timing of events
in the rolling of a key in a DNSSEC-secured zone. It presents
timelines for the key rollover and explicitly identifies the
relationships between the various parameters affecting the process.
Status of this Memo
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This Internet-Draft will expire on January 10, 2013.
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Internet-Draft DNSSEC Key Timing Considerations July 20121. Introduction1.1. Key Rolling Considerations
When a zone is secured with DNSSEC, the zone manager must be prepared
to replace ("roll") the keys used in the signing process. The
rolling of keys may be caused by compromise of one or more of the
existing keys, or it may be due to a management policy that demands
periodic key replacement for security or operational reasons. In
order to implement a key rollover, the keys need to be introduced
into and removed from the zone at the appropriate times.
Considerations that must be taken into account are:
o DNSKEY records and associated information (such as the associated
DS records or RRSIG records created with the key) are not only
held at the authoritative nameserver, they are also cached by
resolvers. The data on these systems can be interlinked, e.g., a
validating resolver may try to validate a signature retrieved from
a cache with a key obtained separately.
o Zone "boot-strapping" events, where a zone is signed for the first
time, can be common in configurations where a large number of
zones are being served. Procedures should be able to cope with
the introduction of keys into the zone for the first time as well
as "steady-state", where the records are being replaced as part of
normal zone maintenance.
o To allow for an emergency re-signing of the zone as soon as
possible after a key compromise has been detected, standby keys
(additional keys over and above those used to sign the zone) need
to be present.
o A query for the DNSKEY RRset returns all DNSKEY records in the
zone. As there is limited space in the UDP packet (even with
EDNS0 support), key records no longer needed must be periodically
removed. (For the same reason, the number of standby keys in the
zone should be restricted to the minimum required to support the
key management policy.)
Management policy, e.g., how long a key is used for, also needs to be
considered. However, the point of key management logic is not to
ensure that a rollover is completed at a certain time but rather to
ensure that no changes are made to the state of keys published in the
zone until it is "safe" to do so ("safe" in this context meaning that
at no time during the rollover process does any part of the zone ever
go bogus). In other words, although key management logic enforces
policy, it may not enforce it strictly.
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A high-level overview of key rollover can be found in
[I-D.ietf-dnsop-rfc4641bis]. In contrast, this document focuses on
the low-level timing detail of two classes of operations described
there, the rollover of key-signing keys, and the rollover of zone
signing keys.
1.2. Types of Keys
Although DNSSEC validation treats all keys equally, [RFC4033]
recognises the broad classification of zone-signing keys (ZSK) and
key-signing keys (KSK). A ZSK is used to authenticate information
within the zone; a KSK is used to authenticate the zone's DNSKEY
RRset. The main implication for this distinction concerns the
consistency of information during a rollover.
During operation, a validating resolver must use separate pieces of
information to perform an authentication. At the time of
authentication, each piece of information may be in its cache or may
need to be retrieved from the authoritative server. The rollover
process needs to happen in such a way that at all times during the
rollover the information is consistent. With a ZSK, the information
is the RRSIG (plus associated RRset) and the DNSKEY. These are both
obtained from the same zone. In the case of the KSK, the information
is the DNSKEY and DS RRset with the latter being obtained from a
different zone.
Although there are similarities in the algorithms to roll ZSKs and
KSKs, there are a number of differences. For this reason, the two
types of rollovers are described separately. It is also possible to
use a single key as both the ZSK and KSK. However, the rolling of
this type of key is not treated in this document.
1.3. Terminology
The terminology used in this document is as defined in [RFC4033] and
[RFC5011].
A number of symbols are used to identify times, intervals, etc. All
are listed in Appendix A.
1.4. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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Internet-Draft DNSSEC Key Timing Considerations July 20122. Rollover Methods2.1. ZSK Rollovers
A ZSK can be rolled in one of three ways:
o Pre-Publication: described in [I-D.ietf-dnsop-rfc4641bis], the new
key is introduced into the DNSKEY RRset which is then re-signed.
This state of affairs remains in place for long enough to ensure
that any cached DNSKEY RRsets contain both keys. At that point
signatures created with the old key can be replaced by those
created with the new key, and the old signatures removed. During
the re-signing process (which may or may not be atomic depending
on how the zone is managed), it doesn't matter which key an RRSIG
record retrieved by a resolver was created with; cached copies of
the DNSKEY RRset will contain both the old and new keys.
Once the zone contains only signatures created with the new key,
there is an interval during which RRSIG records created with the
old key expire from caches. After this, there will be no
signatures anywhere that were created using the old key, and it
can can be removed from the DNSKEY RRset.
o Double-Signature: also mentioned in [I-D.ietf-dnsop-rfc4641bis],
this involves introducing the new key into the zone and using it
to create additional RRSIG records; the old key and existing RRSIG
records are retained. During the period in which the zone is
being signed (again, the signing process may not be atomic),
validating resolvers are always able to validate RRSIGs: any
combination of old and new DNSKEY RRset and RRSIG allows at least
one signature to be validated.
Once the signing process is complete and enough time has elapsed
to allow all old information to expire from caches, the old key
and signatures can be removed from the zone. As before, during
this period any combination of DNSKEY RRset and RRSIG will allow
validation of at least one signature.
o Double-RRSIG: strictly speaking, the use of the term "Double-
Signature" above is a misnomer as the method is not only double
signature, it is also double key as well. A true Double-Signature
method (here called the Double-RRSIG method) involves introducing
new signatures in the zone (while still retaining the old ones)
but not introducing the new key.
Once the signing process is complete and enough time has elapsed
to ensure that all caches that may contain an RR and associated
RRSIG have a copy of both signatures, the key is changed. After a
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further interval during which the old DNSKEY RRset expires from
caches, the old signatures are removed from the zone.
Of three methods, Double-Signature is conceptually the simplest -
introduce the new key and new signatures, then approximately one TTL
later remove the old key and old signatures. Pre-Publication is more
complex - introduce the new key, approximately one TTL later sign the
records, and approximately one TTL after that remove the old key.
Double-RRSIG is essentially the reverse of Pre-Publication -
introduce the new signatures, approximately one TTL later change the
key, and approximately one TTL after that remove the old signatures.
2.2. KSK Rollovers
For ZSKs, the issue for the validating resolver is to ensure that it
has access to the ZSK that corresponds to a particular signature. In
the KSK case, this can never be a problem as the KSK is only used for
one signature (that over the DNSKEY RRset) and both the key and the
signature travel together. Instead, the issue is to ensure that the
KSK is trusted.
Trust in the KSK is either due to the existence of a signed and
validated DS record in the parent zone or an explicitly configured
trust anchor. If the former, the rollover algorithm will need to
involve the parent zone in the addition and removal of DS records, so
timings are not wholly under the control of the zone manager. If the
latter, [RFC5011] timings will be needed to roll the keys. (Even in
the case where authentication is via a DS record, the zone manager
may elect to include [RFC5011] timings in the key rolling process so
as to cope with the possibility that the key has also been explicitly
configured as a trust anchor.)
It is important to note that this does not preclude the development
of key rollover logic; in accordance with the goal of the rollover
logic being able to determine when a state change is "safe", the only
effect of being dependent on the parent is that there may be a period
of waiting for the parent to respond in addition to any delay the key
rollover logic requires. Although this introduces additional delays,
even with a parent that is less than ideally responsive the only
effect will be a slowdown in the rollover state transitions. This
may cause a policy violation, but will not cause any operational
problems.
Like the ZSK case, there are three methods for rolling a KSK:
o Double-Signature: also known as Double-DNSKEY, the new KSK is
added to the DNSKEY RRset which is then signed with both the old
and new key. After waiting for the old RRset to expire from
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caches, the DS record in the parent zone is changed. After
waiting a further interval for this change to be reflected in
caches, the old key is removed from the RRset. (The name "Double-
Signature" is used because, like the ZSK method of the same name,
the new key is introduced and immediately used for signing.)
o Double-DS: the new DS record is published. After waiting for this
change to propagate into caches, the KSK is changed. After a
further interval during which the old DNSKEY RRset expires from
caches, the old DS record is removed.
o Double-RRset: the new KSK is added to the DNSKEY RRset which is
then signed with both the old and new key, and the new DS record
added to the parent zone. After waiting a suitable interval for
the old DS and DNSKEY RRsets to expire from caches, the old DNSKEY
and DS record are removed.
In essence, "Double-Signature" means that the new KSK is introduced
first and used to sign the DNSKEY RRset. The DS record is changed,
and finally the old KSK removed. With "Double-DS" it is the other
way around. Finally, Double-RRset does both updates more or less in
parallel.
2.3. Summary
The methods can be summarised as follows:
+------------------+------------------+-----------------------------+
| ZSK Method | KSK Method | Description |
+------------------+------------------+-----------------------------+
| Pre-Publication | (not applicable) | Publish the DNSKEY before |
| | | the RRSIGs. |
| Double-Signature | Double-Signature | Publish the DNSKEY and |
| | | RRSIGs at same time. For a |
| | | KSK, this happens before |
| | | the DS is published. |
| Double-RRSIG | (not applicable) | Publish RRSIGs before the |
| | | DNSKEY. |
| (not applicable) | Double-DS | Publish DS before the |
| | | DNSKEY. |
| (not applicable) | Double-RRset | Publish DNSKEY and DS in |
| | | parallel. |
+------------------+------------------+-----------------------------+
Table 1
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Internet-Draft DNSSEC Key Timing Considerations July 20123. Key Rollover Timelines3.1. Key States
During the rolling process, a key moves through different states.
The defined states are:
Generated The key has been created, but has not yet been used for
anything.
Published The DNSKEY record - or information associated with it -
is published in the zone, but predecessors of the key (or
associated information) may be held in caches.
The idea of "associated information" is used in rollover
methods where RRSIG or DS records are published first and
the DNSKEY is changed in an atomic operation. It allows
the rollover still to be thought of as moving through a
set of states. In the rest of this section, the term
"key data" should be taken to mean "key or associated
information".
Ready The new key data has been published for long enough to
guarantee that any previous versions of the DNSKEY RRset
have expired from caches.
Active The key has started to be used to sign RRsets. Note that
when this state is entered, it may not be possible for
validating resolvers to use the key for validation in all
cases: the zone signing may not have finished, or the
data might not have reached the resolver because of
propagation delays and/or caching issues. If this is the
case, the resolver will have to rely on the key's
predecessor instead.
Retired A successor key has become active and this key is no
longer being used to generate RRSIGs. However, as there
may still be RRSIGs in caches that were generated using
this key, it is being retained in the zone until they
have expired.
Dead The key is published in the zone but there is no longer
information anywhere that requires its presence. Hence
the key can be removed from the zone at any time.
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Removed The key has been removed from the zone.
There is one additional state, used where [RFC5011] considerations
are in effect (see Section 3.3.4):
Revoked The key is published for a period with the "revoke" bit
set as a way of notifying validating resolvers that have
configured it as an [RFC5011] trust anchor that it is
about to be removed from the zone.
3.2. Zone-Signing Key Timelines
The following sections describe the rolling of a ZSK. They show the
events in the lifetime of a key (referred to as "key N") and cover
its replacement by its successor (key N+1).
3.2.1. Pre-Publication Method
The following diagram shows the timeline of a Pre-Publication
rollover. Time increases along the horizontal scale from left to
right and the vertical lines indicate events in the process.
Significant times and time intervals are marked.
|1| |2| |3| |4| |5| |6| |7| |8| |9|
| | | | | | | | |
Key N | |<-Ipub->|<--->|<-------Lzsk----->|<-Iret->|<--->|
| | | | | | | | |
Key N+1 | | | | |<-Ipub->|<->|<---Lzsk-- - -
| | | | | | | | |
Tgen Tpub Trdy Tact TpubS Tret Tdea Trem
---- Time ---->
Figure 1: Timeline for a Pre-Publication ZSK rollover.
Event 1: Key N is generated at the generate time (Tgen). Although
there is no reason why the key cannot be generated immediately prior
to its publication in the zone (Event 2), some implementations may
find it convenient to create a pool of keys in one operation and draw
from that pool as required. For this reason, it is shown as a
separate event. Keys that are available for use but not published
are said to be generated.
Event 2: Key N's DNSKEY record is put into the zone, i.e., it is
added to the DNSKEY RRset which is then re-signed with the current
key-signing key. The time at which this occurs is the key's
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publication time (Tpub), and the key is now said to be published.
Note that the key is not yet used to sign records.
Event 3: Before it can be used, the key must be published for long
enough to guarantee that any cached version of the zone's DNSKEY
RRset includes this key.
This interval is the publication interval (Ipub) and, for the second
or subsequent keys in the zone, is given by:
Ipub = Dprp + TTLkey
Here, Dprp is the propagation delay - the time taken in the worst-
case situation for a change introduced at the master to replicate to
all slave servers - which depends on the depth of the master-slave
hierarchy. TTLkey is the time-to-live (TTL) for the DNSKEY records
in the zone. The sum is therefore the maximum time taken for
existing DNSKEY records to expire from caches, regardless of the
nameserver from which they were retrieved.
(The case of introducing the first ZSK into the zone is discussed in
Section 3.3.5.)
After a delay of Ipub, the key is said to be ready and could be used
to sign records. The time at which this event occurs is the key's
ready time (Trdy), which is given by:
Trdy = Tpub + Ipub
Event 4: At some later time, the key starts being used to sign
RRsets. This point is the activation time (Tact) and after this, the
key is said to be active.
Event 5: At some point thought must be given to its successor (key
N+1). As with the introduction of the currently active key into the
zone, the successor key will need to be published at least Ipub
before it is activated. Denoting the publication time of the
successor key by TpubS, then:
TpubS <= Tact + Lzsk - Ipub
Here, Lzsk is the length of time for which a ZSK will be used (the
ZSK lifetime). It should be noted that unlike the publication
interval, Lzsk is not determined by timing logic, but by key
management policy. Lzsk will be set by the operator according to
their assessment of the risks posed by continuing to use a key and
the risks associated with key rollover. However, operational
considerations may mean a key is active for slightly more or less
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than Lzsk.
Event 6: While key N is still active, its successor becomes ready.
From this time onwards, key N+1 could be used to sign the zone.
Event 7: When key N has been in use for an interval equal to the ZSK
lifetime, it is retired (i.e., it will never again be used to
generate new signatures) and key N+1 activated and used to sign the
zone. This is the retire time of key N (Tret) and is given by:
Tret = Tact + Lzsk
It is also the activation time of the successor key (TactS). Note
that operational considerations may cause key N to remain in use for
longer than Lzsk; if so, the retirement actually occurs when the
successor key is made active.
Event 8: The retired key needs to be retained in the zone whilst any
RRSIG records created using this key are still published in the zone
or held in caches. (It is possible that a validating resolver could
have an unexpired RRSIG record and an expired DNSKEY RRset in the
cache when it is asked to provide both to a client. In this case the
DNSKEY RRset would need to be looked up again.) This means that once
the key is no longer used to sign records, it should be retained in
the zone for at least the retire interval (Iret) given by:
Iret = Dsgn + Dprp + TTLsig
Dsgn is the delay needed to ensure that all existing RRsets have been
re-signed with the new key. Dprp is (as described above) the
propagation delay, required to guarantee that the updated zone
information has reached all slave servers, and TTLsig is the maximum
TTL of all the RRSIG records in the zone created with the ZSK.
The time at which all RRSIG records created with this key have
expired from resolver caches is the dead time (Tdea), given by:
Tdea = Tret + Iret
... at which point the key is said to be dead.
Event 9: At any time after the key becomes dead, it can be removed
from the zone's DNSKEY RRset, which must then be re-signed with the
current key-signing key. This time is the removal time (Trem), given
by:
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Trem >= Tdea
... at which time the key is said to be removed.
3.2.2. Double-Signature Method
The timeline for a double-signature rollover is shown below. The
diagram follows the convention described in Section 3.2.1
|1| |2| |3| |4| |5|
| | | | |
Key N | |<----Lzsk--->|<---Iret--->| |
| | | | |
Key N+1 | | |<-----Lzsk------- - -
| | | | |
Tgen Tact Tret Tdea Trem
---- Time ---->
Figure 2: Timeline for a Double-Signature ZSK rollover.
Event 1: Key N is generated at the generate time (Tgen). Although
there is no reason why the key cannot be generated immediately prior
to its publication in the zone (Event 2), some implementations may
find it convenient to create a pool of keys in one operation and draw
from that pool as required. For this reason, it is shown as a
separate event. Keys that are available for use but not published
are said to be generated.
Event 2: Key N is added to the DNSKEY RRset and is then used to sign
the zone; existing signatures in the zone are not removed. This is
the activation time (Tact), after which the key is said to be active.
Event 3: After the current key (key N) has been in use for its
intended lifetime (Lzsk), the successor key (key N+1) is introduced
into the zone and starts being used to sign RRsets: neither the
current key nor the signatures created with it are removed. The
successor is key is now active and the current key is said to be
retired. This time is the retire time of the key (Tret); it is also
the activation time of the successor key (TactS).
Tret = Tact + Lzsk
Event 4: Before key N can be withdrawn from the zone, all RRsets that
need to be signed must have been signed by the successor key (key
N+1) and any old RRsets that do not include the new key or new RRSIGs
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must have expired from caches. Note that the signatures are not
replaced - each RRset is signed by both the old and new key.
This takes Iret, the retire interval, given by the expression:
Iret = Dsgn + Dprp + max(TTLkey, TTLsig)
As before, Dsgn is the delay needed to ensure that all existing
RRsets have been signed with the new key and Dprp is the propagation
delay. The final term (the maximum of TTLkey and TTLsig) is the
period to wait for key and signature data associated with key N to
expire from caches. (TTLkey is the TTL of the DNSKEY RRset and
TTLsig is the maximum TTL of all the RRSIG records in the zone
created with the ZSK. The two may be different: although the TTL of
an RRSIG is equal to the TTL of the RRs in the associated RRset
[RFC4034], the DNSKEY RRset only needs to be signed with the KSK.)
At the end of this interval, key N is said to be dead. This occurs
at the dead time (Tdea) so:
Tdea = Tret + Iret
Event 5: At some later time key N and the signatures generated with
it can be removed from the zone. This is the removal time Trem,
given by:
Trem >= Tdea
3.2.3. Double-RRSIG Method
The timeline for a double-signature rollover is shown below. The
diagram follows the convention described in Section 3.2.1
|1||2| |3| |4||5| |6| |7||8| |9| |10|
| | | | | | | | | |
Key N | |<-Dsgn->| | |<--------Lzsk-------->|<-Iret->| |
| |<---IpubG-->| | | | | | |
| | | | | | | | | |
Key N+1 | | | | | |<-IpubG->| | | |
| | | | | | | | | |
Tgen Tpub Trdy Tact TpubS TrdyS Tret Tdea Trem
---- Time ---->
Figure 3: Timeline for a Double-Signature ZSK rollover.
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Event 1: Key N is generated at the generate time (Tgen). Although
there is no reason why the key cannot be generated immediately prior
to its publication in the zone (Event 2), some implementations may
find it convenient to create a pool of keys in one operation and draw
from that pool as required. For this reason, it is shown as a
separate event. Keys that are available for use but not published
are said to be generated.
Event 2: Key N is used to sign the zone but existing signatures are
retained. Although the new ZSK is not published in the zone at this
point, for analogy with the other ZSK rollover methods and because
this is the first time that key information is visible (albeit
indirectly through the created signatures) this time is called the
publication time (Tpub).
Event 3: After the signing interval, Dsgn, all RRsets that need to be
signed have been signed by the new key. (As a result, all these
RRsets are now signed twice, once by the (still-absent) key N and
once by its predecessor.
Event 4: There is now a delay while the old signature information
expires from caches. This interval is given by the expression:
Dprp + TTLsig
As before, Dprp is the propagation delay and TTLsig is the maximum
TTL of all the RRSIG records in the zone created with the ZSK.
Again in analogy with other key rollover methods, this is defined as
key N's ready time (Trdy) and the key is said to be in the ready
state. (Although the key is not in the zone, it is ready to be
used.) The interval between the publication and ready times is the
publication interval of the signature, IpubG, i.e.,
Trdy = Tpub + IpubG
where
IpubG = Dsgn + Dprp + TTLsig
Event 5: At some later time the predecessor key is removed and the
key N added to the DNSKEY RRset. As all the signed RRs have
signatures created by the old and new keys, the records can still be
authenticated. This time is the activation time (Tact) and the key
is now said to be active.
Event 6: At some point thought must be given to rolling the key. The
first step is to publish signatures created by the successor key (key
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N+1) early enough for key N to be replaced after it has been active
for its scheduled lifetime. This occurs at TpubS (the publication
time of the successor), given by:
TpubS <= Tact + Lzsk - IpubG
Event 7: The signatures have propagated and the new key could be
added to the zone. This time is the ready time of the successor key
(TrdyS).
TrdyS = TpubS + IpubG
... where IpubG is as defined above.
Event 8: At some later time key N is removed from the zone's DNSKEY
RRset and the successor key (key N+1) is added to it. This is the
retire time of the key (Tret).
Event 9: The signatures must remain in the zone for long enough that
the new DNSKEY RRset has had enough time to propagate to all caches.
Once caches contain the new DNSKEY, the old signatures are no longer
of use and can be considered to be dead as they cannot be validated
by any key. In analogy with other rollover methods, the time at
which this occurs is the dead time (Tdea), given by:
Tdea = Tret + Iret
... where Iret is the retire interval, given by:
Iret = Dprp + TTLkey
Dprp is as defined earlier and TTLkey is the TTL of the DNSKEY RRset.
Event 10: At some later time the signatures can be removed from the
zone. In analogy with other rollover methods, this time is called
the remove time (Trem) and is given by:
Trem >= Tdea
3.3. Key-Signing Key Rollover Timelines
The following sections describe the rolling of a KSK. They show the
events in the lifetime of a key (referred to as "key N") and cover it
replacement by its successor (key N+1).
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The timeline for a double-signature rollover is shown below. The
diagram follows the convention described in Section 3.2.1
|1| |2| |3| |4| |5|
| | | | |
Key N | |<-Ipub->|<--->|<-Dreg->|<-----Lksk--- - -
| | | | |
Key N+1 | | | | |
| | | | |
Tgen Tpub Trdy Tsub Tact
---- Time ---->
(continued ...)
|6| |7| |8| |9| |10| |11|
| | | | | |
Key N - - -------------Lksk------->|<-Iret->| |
| | | | | |
Key N+1 |<-Ipub->|<--->|<-Dreg->|<--------Lksk----- - -
| | | | | |
TpubS TrdyS TsubS Tret Tdea Trem
---- Time (cont) ---->
Figure 4: Timeline for a Double-Signature KSK rollover.
Event 1: Key N is generated at the generate time (Tgen). Although
there is no reason why the key cannot be generated immediately prior
to its publication in the zone (Event 2), some implementations may
find it convenient to create a pool of keys in one operation and draw
from that pool as required. For this reason, it is shown as a
separate event. Keys that are available for use but not published
are said to be generated.
Event 2: Key N is introduced into the zone; it is added to the DNSKEY
RRset, which is then signed by key N and all currently active KSKs.
(So at this point, the DNSKEY RRset is signed by both key N and its
predecessor KSK. If other KSKs were active, it is signed by these as
well.) This is the publication time (Tpub); after this the key is
said to be published.
Event 3: Before it can be used, the key must be published for long
enough to guarantee that any validating resolver that has a copy of
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the DNSKEY RRset in its cache will have a copy of the RRset that
includes this key: in other words, that any prior cached information
about the DNSKEY RRset has expired.
The interval is the publication interval (Ipub) and, for the second
or subsequent KSKs in the zone, is given by:
Ipub = DprpC + TTLkey
... where DprpC is the propagation delay for the child zone (the zone
containing the KSK being rolled) and TTLkey the TTL for the DNSKEY
RRset. The time at which this occurs is the key's ready time, Trdy,
given by:
Trdy = Tpub + Ipub
(The case of introducing the first KSK into the zone is discussed in
Section 3.3.5.)
Event 4: At some later time, the DS record corresponding to the new
KSK is submitted to the parent zone for publication. This time is
the submission time, Tsub.
Event 5: The DS record is published in the parent zone. As this is
the point at which all information for authentication - both DNSKEY
and DS record - is available in the two zones, in analogy with other
rollover methods, this is called the activation time of the key
(Tact):
Tact = Tsub + Dreg
... where Dreg is the registration delay, the time taken after the DS
record has been received by the parent zone manager for it to be
placed in the zone. (Parent zones are often managed by different
entities, and this term accounts for the organisational overhead of
transferring a record.)
Event 6: While key N is active, thought needs to be given to its
successor (key N+1). At some time before the scheduled end of the
KSK lifetime, the successor KSK is published in the zone. (As
before, this means that the DNSKEY RRset is signed by both the
current and successor KSK.) This time is the publication time of the
successor key, TpubS, given by:
TpubS <= Tact + Lksk - Dreg - Ipub
... where Lksk is the scheduled lifetime of the KSK.
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Event 7: After an interval Ipub, key N+1 becomes ready (in that all
caches that have a copy of the DNSKEY RRset have a copy of this key).
This time is the ready time of the successor (TrdyS).
Event 8: At the submission time of the successor (TsubS), the DS
record corresponding to key N+1 is submitted to the parent zone.
Event 9: The successor DS record is published in the parent zone and
the current DS record withdrawn. The current key is said to be
retired and the time at which this occurs is Tret, given by:
Tret = Tact + Lksk
Event 10: Key N must remain in the zone until any caches that contain
a copy of the DS RRset have a copy containing the new DS record.
This interval is the retire interval, given by:
Iret = DprpP + TTLds
... where DprpP is the propagation delay in the parent zone and TTLds
the TTL of a DS record in the parent zone.
As the key is no longer used for anything, is said to be dead. This
point is the dead time (Tdea), given by:
Tdea = Tret + Iret
Event 11: At some later time, key N is removed from the zone's DNSKEY
RRset (at the remove time Trem); the key is now said to be removed.
Trem >= Tdea
3.3.2. Double-DS Method
The timeline for a double-DS rollover is shown below. The diagram
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|1| |2| |3| |4| |5| |6|
| | | | | |
Key N | |<-Dreg->|<-IpubP->|<-->|<---------Lksk------- - -
| | | | | |
Key N+1 | | | | |<---->|<--Dreg+IpubP- - -
| | | | | |
Tgen Tsub Tpub Trdy Tact TsubS
---- Time ---->
(continued ...)
|7| |8| |9| |10|
| | | |
Key N - - -----Lksk---------->|<-Iret->| |
| | | |
Key N+1 - - --Dreg+IpubP->|<--->|<------Lksk------ - -
| | | |
TrdyS Tret Tdea Trem
---- Time ---->
Figure 5: Timeline for a Double-DS KSK rollover.
Event 1: Key N is generated at the generate time (Tgen). Although
there is no reason why the key cannot be generated immediately prior
to its publication in the zone (Event 2), some implementations may
find it convenient to create a pool of keys in one operation and draw
from that pool as required. For this reason, it is shown as a
separate event. Keys that are available for use but not published
are said to be generated.
Event 2: The DS RR is submitted to the parent zone for publication.
This time is the submission time, Tsub.
Event 3: After the registration delay, Dreg, the DS record is
published in the parent zone. This is the publication time Tpub,
given by:
Tpub = Tsub + Dreg
Event 4: At some later time, any cache that has a copy of the DS
RRset will have a copy of the DS record for key N. At this point, key
N, if introduced into the DNSKEY RRset, could be used to validate the
zone. For this reason, this time is known as the key's ready time,
Trdy, and is given by:
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Trdy = Tpub + IpubP
IpubP is the parent publication interval and is given by the
expression:
IpubP = DprpP + TTLds
... where DprpP is the propagation delay for the parent zone and
TTLds the TTL assigned to DS records in that zone.
Event 5: At some later time, the key rollover takes place and the new
key (key N) is introduced into the DNSKEY RRset and used to sign
that.
As both the old and new DS records have been in the parent zone long
enough to ensure that they are in caches that contain the DS RRset,
the zone can be authenticated throughout the rollover - the
validating resolver either has a copy of the DNSKEY RRset
authenticated by the predecessor key, or it has a copy of the updated
RRset authenticated with the new key.
This time is key N's activation time (Tact) and at this point the key
is said to be active.
Event 6: At some point thought must be given to key replacement. The
DS record for the successor key must be submitted to the parent zone
at a time such that when the current key is withdrawn, any cache that
contains the zone's DS records has data about the DS record of the
successor key. The time at which this occurs is the submission time
of the successor, given by:
TsubS <= Tact + Lksk - IpubP - Dreg
... where Lksk is the lifetime of key N according to policy.
Event 7: The successor key (key N+1) enters the ready state, i.e.,
its DS record is now in caches that contain the parent DS RRset.
This is the ready time of the successor key, TrdyS.
(The interval between events 6 and 7 for the key N+1 correspond to
the interval between events 2 and 4 for key N)
Event 8: When key N has been active for its lifetime (Lksk), it is
replaced in the DNSKEY RRset by key N+1; the RRset is then signed
with the new key. This is the retire time (Tret) of key N, given by:
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Tret = Tact + Lksk
Event 9: At some later time, all copies of the old DNSKEY RRset have
expired from caches and the old DS record is no longer needed. In
analogy with other rollover methods, this is called the dead time,
Tdea, and is given by:
Tdea = Tret + Iret
... where Iret is the retire interval, given by:
Iret = DprpC + TTLkey
As before, this term includes DprpC, the time taken to propagate the
RRset change through the master-slave hierarchy of the child zone and
TTLkey, the time taken for the DNSKEY RRset to expire from caches.
Event 10: At some later time, the DS record is removed from the
parent zone. In analogy with other rollover methods, this is the
removal time (Trem), given by:
Trem >= Tdea
3.3.3. Double-RRset Method
The timeline for a double-RRset rollover is shown below. The diagram
follows the convention described in Section 3.2.1
|1| |2| |3| |4| |5| |6|
| | | | | |
Key N | |<-Ipub->|<-----Lksk----->| |
| | | | | |
Key N+1 | | | |<-Ipub->| |
| | | | | |
Tgen Tpub Tact TpubS Tret Trem
---- Time ---->
Figure 6: Timeline for a Double-RRset KSK rollover.
Event 1: Key N is generated at the generate time (Tgen). Although
there is no reason why the key cannot be generated immediately prior
to its publication in the zone (Event 2), some implementations may
find it convenient to create a pool of keys in one operation and draw
from that pool as required. For this reason, it is shown as a
separate event. Keys that are available for use but not published
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are said to be generated.
Event 2: The key is added to and used for signing the DNSKEY RRset
and is thereby published in the zone. At the same time the
corresponding DS record is submitted to the parent zone for
publication. This time is the publish time (Tpub) and the key is now
said to be published.
Event 3: At some later time, the DS record is published in the parent
zone and at some time after that, the updated information has reached
all caches: any cache that holds a DNSKEY RRset from the child zone
will have a copy that includes the new KSK, and any cache that has a
copy of the parent DS RRset will have a copy that includes the new DS
record.
The time at which this occurs is called the activation time of the
new KSK (Tact), given by:
Tact = Tpub + Ipub
... where Ipub is the publication interval, given by:
Ipub = max(IpubP, IpubC),
IpubP being the publication interval in the parent zone and IpubC the
publication interval in the child zone. The parent zone's
publication interval is given by:
IpubP = Dreg + DprpP + TTLds
where Dreg is the registration delay, the time taken for the DS
record to be published in the parent zone. DprpP is the parent
zone's propagation delay and TTLds is the TTL of the DS record in
that zone.
The child's publication interval is given by a similar equation:
IpubC = DprpC + TTLkey
... where DprpC is the propagation delay in the child zone and TTLkey
the TTL of a DNSKEY record.
Event 4: At some point we need to give thought to key replacement.
The successor key (key N+1) must be introduced into the zone (and its
DS record submitted to the parent) at a time such that it becomes
active when the current key has been active for its lifetime, Lksk.
This time is TpubS, the publication time of the successor key, and is
given by:
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TpubS <= Tact + Lksk - Ipub
... where Lksk is the lifetime of the KSK.
Event 5: Key N+1's DNSKEY and DS records are in any caches that
contain the child zone DNSKEY and/or the parent zone DS RR, and so
the zone can be validated with the new key. This is the activation
time of the successor key (TactS) and by analogy with other rollover
methods, it is also the retire time of the current key. Since at
this time the zone can be validated by the successor key, there is no
reason to keep the current key in the zone and the time can also be
regarded as the current key's dead time. Thus:
Tret = Tdea = TactS = Tact + Lksk
Event 6: At some later time, the key N's DS and DNSKEY records are
removed from their respective zones. In analogy with other rollover
methods, this is the removal time (Trem), given by:
Trem >= Tdea
3.3.4. Interaction with Configured Trust Anchors
Although the preceding sections have been concerned with rolling KSKs
where the trust anchor is a DS record in the parent zone, zone
managers may want to take account of the possibility that some
validating resolvers may have configured trust anchors directly.
Rolling a configured trust anchor is dealt with in [RFC5011]. It
requires introducing the KSK to be used as the trust anchor into the
zone for a period of time before use, and retaining it (with the
"revoke" bit set) for some time after use.
3.3.4.1. Addition of KSK
When the new key is introduced, the publication interval (Ipub) in
the Double-Signature and Double-RRset methods should also be subject
to the condition:
Ipub >= Dprp + max(Itrp, TTLkey)
... where the right hand side of the expression is the time taken for
the change to propagate to all nameservers for the zone plus the
"trust point" interval. This latter term is the interval required to
guarantee that a resolver configured for the automatic update of keys
from a particular trust point will see at least two validated DNSKEY
RRsets containing the new key (a requirement from [RFC5011], section2.4.1). It is defined by the expression:
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Itrp >= (2 * queryInterval) + (n * retryTime)
... where queryInterval and retryTime are as defined in section 2.3
of [RFC5011]. "n" is the total number of retries needed by the
resolver during the two attempts to get the DNSKEY RRset.
The first term of the expression (2 * queryInterval) represents the
time to obtain two validated DNSKEY RRsets. The second term (n *
retryTime) is a safety margin, with the value of "n" reflecting the
degree of confidence in the communication between a resolver and the
trust point.
In the Double-DS method, instead of swapping the KSK RRs in a single
step, there must now be a period of overlap. In other words, the new
KSK must be introduced into the zone at least:
DprpC + max(Itrp, TTLkey)
... before the switch is made.
3.3.4.2. Removal of KSK
The timeline for the removal of the key in all methods is modified by
introducing a new state, "revoked". When the key reaches its dead
time, instead of being declared "dead", it is revoked; the "revoke"
bit is set in the published DNSKEY RR, and the DNSKEY RRset re-signed
with the current and revoked keys. The key is maintained in this
state for the "revoke" interval, Irev, given by:
Irev >= 30 days
... 30 days being the [RFC5011] remove hold-down time. After this
time, the key is dead and can be removed from the zone.
3.3.5. Introduction of First Keys
There are no timing considerations associated with the introduction
of the first keys into a zone other that they must be introduced and
the zone validly signed before a chain of trust to the zone is
created.
This is important: in the case of a secure parent, it means ensuring
that the DS record is not published in the parent zone until there is
no possibility that a validating resolver can obtain the record yet
is not able to obtain the corresponding DNSKEY. In the case of an
insecure parent, i.e., the initial creation of a new security apex,
it is not possible to guarantee this. It is up to the operator of
the validating resolver to wait for the new KSK to appear at all
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servers for the zone before configuring the trust anchor.
4. Standby Keys
Although keys will usually be rolled according to some regular
schedule, there may be occasions when an emergency rollover is
required, e.g., if the active key is suspected of being compromised.
The aim of the emergency rollover is to allow the zone to be re-
signed with a new key as soon as possible. As a key must be in the
ready state to sign the zone, having at least one additional key (a
standby key) in this state at all times will minimise delay.
In the case of a ZSK, a standby key only really makes sense with the
Pre-Publication method. A permanent standby DNSKEY RR should be
included in the zone or successor keys could be introduced as soon as
possible after a key becomes active. Either way results in one or
more additional ZSKs in the DNSKEY RRset that can immediately be used
to sign the zone if the current key is compromised.
(Although in theory the mechanism could be used with both the Double-
Signature and Double-RRSIG methods, it would require pre-publication
of the signatures. Essentially, the standby key would be permanently
active, as it would have to be periodically used to renew signatures.
Zones would also permanently require two sets of signatures.)
It is also possible to have a standby KSK. The Double-Signature
method requires that the standby KSK be included in the DNSKEY RRset;
rolling the key then requires just the introduction of the DS record
in the parent. Note that the standby KSK should also be used to sign
the DNSKEY RRset. As the RRset and its signatures travel together,
merely adding the KSK without using it to sign the DNSKEY RRset does
not provide the desired time saving: for a KSK to be used in a
rollover the DNSKEY RRset must be signed with it, and this would
introduce a delay while the old RRset (not signed with the new key)
expires from caches.
The idea of a standby KSK in the Double-RRset rollover method
effectively means having two active keys (as the standby KSK and
associated DS record would both be published at the same time in
their respective zones).
Finally, in the Double-DS method of rolling a KSK, it is not a
standby key that is present, it is a standby DS record in the parent
zone.
Whatever algorithm is used, the standby item of data can be included
in the zone on a permanent basis, or be a successor introduced as
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early as possible.
5. Algorithm Considerations
The preceding sections have implicitly assumed that all keys and
signatures are created using a single algorithm. However, [RFC4035]
(section 2.4) states that "There MUST be an RRSIG for each RRset
using at least one DNSKEY of each algorithm in the zone apex DNSKEY
RRset".
Except in the case of an algorithm rollover - where the algorithms
used to create the signatures are being changed - there is no
relationship between the keys of different algorithms. This means
that they can be rolled independently of one another. In other
words, the key rollover logic described above should be run
separately for each algorithm; the union of the results is included
in the zone, which is signed using the active key for each algorithm.
6. Limitation of Scope
This document represents current thinking at the time of publication.
However, the subject matter is evolving and it is more than likely
that this document will need to be revised in the future.
Some of the techniques and ideas that DNSSEC operators are
considering differ from this those described in this document. Of
particular interest are alternatives to the strict split into KSK and
ZSK key roles and the consequences for rollover logic from partial
signing (i.e., when the new key initially only signs a fraction of
the zone while leaving other signatures generated by the old key in
place).
Furthermore, as noted in section 5, this document covers only rolling
keys of the same algorithm: it does not cover transitions between
algorithms. The timing issues associated with algorithm rollovers
will require a separate document.
The reader is therefore reminded that DNSSEC is, as of date of
publication, in the early stages of deployment, and best practices
may further develop over time.
7. Summary
For ZSKs, "Pre-Publication" is generally considered to be the
preferred way of rolling keys. As shown in this document, the time
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taken to roll is wholly dependent on parameters under the control of
the zone manager.
In contrast, "Double-RRset" is the most efficient method for KSK
rollover due to the ability to have new DS records and DNSKEY RRsets
propagate in parallel. The time taken to roll KSKs may depend on
factors related to the parent zone if the parent is signed. For
zones that intend to comply with the recommendations of [RFC5011], in
virtually all cases the rollover time will be determined by the
RFC5011 "add hold-down" and "remove hold-down" times. It should be
emphasized that this delay is a policy choice and not a function of
timing values and that it also requires changes to the rollover
process due to the need to manage revocation of trust anchors.
Finally, the treatment of emergency key rollover is significantly
simplified by the introduction of standby keys as standard practice
during all types of rollovers.
8. IANA Considerations
This memo includes no request to IANA.
9. Security Considerations
This document does not introduce any new security issues beyond those
already discussed in [RFC4033], [RFC4034], [RFC4035] and [RFC5011].
10. Acknowledgements
The authors gratefully acknowledge help and contributions from Roy
Arends, Matthijs Mekking and Wouter Wijngaards.
11. Normative References
[I-D.ietf-dnsop-rfc4641bis]
Kolkman, O. and M. Mekking, "DNSSEC Operational Practices,
Version 2", draft-ietf-dnsop-rfc4641bis-11 (work in
progress), April 2012.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
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[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[RFC5011] StJohns, M., "Automated Updates of DNS Security (DNSSEC)
Trust Anchors", RFC 5011, September 2007.
Appendix A. List of Symbols
The document defines a number of symbols, all of which are listed
here. All are of the form:
All symbols used in the text are of the form:
<TYPE><id><INST>
where:
<TYPE> is an upper-case character indicating what type the symbol is.
Defined types are:
D delay: interval that is a feature of the process
I interval between two events
L lifetime: interval set by the zone manager
T a point in time
TTL TTL of a record
I and T and TTL are self-explanatory. Like I, both D and L are time
periods, but whereas I values are intervals between two events (even
if the events are defined in terms of the interval, e.g., the dead
time occurs "retire interval" after the retire time), D and L are
fixed intervals: a "D" interval (delay) is a feature of the process,
probably outside control of the zone manager, whereas an "L" interval
(lifetime) is chosen by the zone manager and is a feature of policy.
<id> is lower-case and defines what object or event the variable is
related to, e.g.,
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act activation
pub publication
ret retire
Finally, <INST> is a capital letter that distinguishes between the
same variable applying to different instances of an object and is one
of:
C child
G signature
P parent
S successor
The list of variables used in the text is:
Dprp Propagation delay. The amount of time for a change made at
a master nameserver to propagate to all the slave
nameservers.
DprpC Propagation delay in the child zone.
DprpP Propagation delay in the parent zone.
Dreg Registration delay: the time taken for a DS record
submitted to a parent zone to appear in it. As a parent
zone is often managed by a different organisation to that
managing the child zone, the delays associated with passing
data between zones is captured by this term.
Dsgn Signing delay. After the introduction of a new ZSK, the
amount of time taken for all the RRs in the zone to be
signed with it.
Ipub Publication interval. The amount of time that must elapse
after the publication of a key before it can be assumed
that any resolvers that have the DNSKEY RRset cached have a
copy of this key.
IpubC Publication interval in the child zone.
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IpubG Publication interval for the signature created by a ZSK:
the amount of time that must elapse after the signature has
been created before it can be assumed that any resolves
that have the RRset and RRSIG cached have a copy of this
signature.
IpubP Publication interval in the parent zone.
Iret Retire interval. The amount of time that must elapse after
a key enters the retire state for any signatures created
with it to be purged from validating resolver caches.
Irev Revoke interval. The amount of time that a KSK must remain
published with the revoke bit set to satisfy [RFC5011]
considerations.
Itrp Trust-point interval. The amount of time that a trust
anchor must be published for to guarantee that a resolver
configured for an automatic update of keys will see the new
key at least twice.
Lksk Lifetime of a key-signing key. This is the intended amount
of time for which this particular KSK is regarded as the
active KSK. Depending on when the key is rolled-over, the
actual lifetime may be longer or shorter than this.
Lzsk Lifetime of a zone-signing key. This is the intended
amount of time for which the ZSK is used to sign the zone.
Depending on when the key is rolled-over, the actual
lifetime may be longer or shorter than this.
Tact Activation time of the key; the time at which the key is
regarded as the principal key for the zone.
TactS Activation time of the successor key.
Tdea Dead time of a key. Applicable only to ZSKs, this is the
time at which any record signatures held in validating
resolver caches are guaranteed to be created with the
successor key.
Tgen Generate time of a key. The time that a key is created.
Tpub Publication time of a key. The time that a key appears in
a zone for the first time.
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TpubS Publication time of the successor key.
Trem Removal time of a key. The time at which a key is removed
from the zone.
Tret Retire time of a key. The time at which a successor key
starts being used to sign the zone.
Trdy Ready time of a key. The time at which it can be
guaranteed that validating resolvers that have key
information from this zone cached have a copy of this key
in their cache. (In the case of KSKs, should the
validating resolvers also have DS information from the
parent zone cached, the cache must include information
about the DS record corresponding to the key.)
TrdyS Ready time of a successor key.
Tsub Submission time - the time at which the DS record of a KSK
is submitted to the parent.
TsubS Submission time of the successor key.
TTLds Time to live of a DS record (in the parent zone).
TTLkey Time to live of a DNSKEY record. (By implication, this is
also the time to live of the signatures on the DNSKEY
RRset.)
TTLsig The maximum time to live of all the RRSIG records in the
zone that were created with the ZSK.
Appendix B. Change History (To be removed on publication)
o draft-ietf-dnsop-dnssec-key-timing-03
* Clarifications of and corrections to wording (Marc Lampo, Alfred
Hoenes).
* Updated timings related to trust anchor interaction (Matthijs
Mekking).
* Updated RFC 4641 reference to 4641bis (Alfred Hoenes).
* Moved change history to end of document (Alfred Hoenes).
o draft-ietf-dnsop-dnssec-key-timing-02
* Significant re-wording of some sections.
* Removal of events noting change of state of predecessor key from
ZSK Double-RRSIG and Double-Signature methods.
* Change order of bullet points (and some wording) in section 1.1.
Morris, et al. Expires January 10, 2013 [Page 31]